1. Field of the Invention
The present invention relates to a device for direct or indirect application of a liquid or viscous coating medium onto a moving material web, specifically a paper or cardboard web.
2. Description of the Related Art
Devices for direct or indirect coating of a moving material web are generally known in the current state of the art. Reference can be made to the VOITH SULZER article "Advances in Coating Equipment" by Dr. M. Kustermann. This article may be acquired from the assignee under order number P3008.
The process of coating medium application by use of an applicator unit onto the moving material web is generally influenced by a series of disturbing influences. Even when the coating medium is supplied to the material web, irregular air flows across the operating width of the material web that is to be coated may negatively influence the coating result. Particularly at high operating speeds, these air currents originate from the boundary air layer that is carried along by the material web on its surface. It also has a negative effect on the coating result if the absorption capacity of the material web that is to be coated varies in its longitudinal or flow direction, or in its cross direction. This specifically addresses the absorption capacity of the material web, which is at least one factor responsible for how quickly the coating medium dewaters into the material web. It is also detrimental if, based on the given physical characteristics such as surface tension and viscose elasticity, undesirable structures occur in the applied medium. In indirect application methods, there are additional irregularities in the surface structure of the coated material web, resulting for example, from the so-called film splitting effect. In direct application methods--for example when utilizing an open jet nozzle coater with a downstream doctoring device, a "Spray Coating" coater, a "Curtain Coating" coater or similar problems may develop. These problems include effects such as doctoring streaks, grooving when utilizing profiled metering rods, ink splashes or film splitting when utilizing smooth metering rods, and metering rod flooding (this term is generally used for excessive build up of coating medium at the end of a metering rod), which may negatively influence the coating result.